Roseiflexus castenholzii is a heterotrophic, thermophilic, filamentous anoxygenetic phototroph (FAP) bacterium.

[2][3] R. castenholzii was first isolated from red-colored bacterial mats located Nakabusa hot springs in Japan.

[1] Because this organism is a phototroph, it utilizes photosynthesis to fix carbon dioxide and build biomolecules.

[4] In contrast to most other FAPs, R. castenholzii does not have chlorosomes, which contain great amounts of photosynthetic pigments.

[4] Because chlorosomes can obstruct observations of photosynthetic complexes, Roseiflexus castenholzii is considered a model organism to study the reaction centers FAPs have.

[4] The reaction center in Roseiflexus castenholzii is closely related to the RC of Chloroflexus aurantiacus.